Polyarylene sulfide typified by polyphenylene sulfide (this may hereinafter be abbreviated as “PPS”) is a resin having properties suitable as engineering plastics such as excellent heat resistance, barrier properties, chemical resistance, electrically insulating property, wet heat resistance, and flame retardancy. It can be molded into various molded components, films, sheets, fibers and so on by injection molding or extrusion forming and has been used widely in fields where heat resistance and chemical resistance are required, such as various electric/electronic components, machine components and automotive components.
As a concrete method of producing the polyarylene sulfide and as an industrial production method, methods comprising causing an alkali metal sulfide such as sodium sulfide to react with a polyhalogenated aromatic compound such as p-dichlorobenzene in an organic amide solvent such as N-methyl-2-pyrrolidone have been used widely. However, that production method needs the reaction to be carried out at high temperature under high pressure under strong alkali conditions. Moreover, that method has a large process cost problem because it is an energy-intensive process that needs an expensive high-boiling polar solvent such as N-methylpyrrolidone and has a large cost for solvent recovery.
A commercially available polyarylene sulfide obtained by this method contains chlorine in an amount of about 2000 to about 4000 ppm and alkali metal in an amount of about 1000 to about 3000 ppm at the terminals thereof. That deterioration in physical properties such as electric characteristics is caused by the presence of alkali metal salts in a polymer often becomes a problem when the polymer is applied to the field of electric/electronic parts.
Moreover, the polyarylene sulfide obtained by that method is a polymer not high enough in molecular weight to be used for molding applications and has a very wide molecular weight distribution (Mw/Mn) of from 5.0 to 20 and therefore, the use thereof for molding applications has caused some problems, e.g., sufficient mechanical characteristics are not developed, or much gas component is generated when having been heated, or much component is eluted when having come into contact with a solvent. Therefore, for example, it is necessary to further perform a step of increasing molecular weight such as thermal oxidative crosslinking in the air, and this renders the process complicated and lowers productivity (e.g., JP 45-3368-B (pages 7 to 10)).
Moreover, polyarylene sulfides are engineering plastics excellent in heat resistance, chemical resistance, and flame retardancy and superior in mechanical properties, but they are problematic in that the compatibility with other resins is low and the paintability to molding articles is also low.
For the purpose of improving such problems with polyarylene sulfides, including paintability and compatibility with other resins, many methods of introducing functional groups into polyarylene sulfides are known. For example, there is a method that comprises melt-kneading a polyarylene sulfide prepared by a conventional reaction with a compound having a functional group such as an alkali metal salt of an organic compound and maleic anhydride (for example, JP 11-286548-A (pages 4 to 8) and JP 2-283763-A (pages 2 to 9)).
Alternatively, there is a method that comprises introducing functional groups into a polymer main chain by copolymerizing a functional group-containing polyhalo compound in polymerizing a polyarylene sulfide (e.g., JP 7-102064-A (pages 2 to 6)).
In both the methods, however, an attempt to introduce a sufficiently effective functional group renders the operation complicated. Moreover, the above-described problems with polyarylene sulfides such as gas generation amount, alkali metal content, and chlorine content, have not been solved by those methods.
On the other hand, a method of producing a polyarylene sulfide by heating a cyclic polyarylene sulfide has been disclosed as another method of producing a polyarylene sulfide. That method is expected to afford a polyarylene sulfide that has a high molecular weight and a narrow molecular weight distribution and that exhibits small weight loss when being heated (e.g., WO 2007/034800 (pages 40 to 41) and Polymer, Vol. 37, No. 14, 1996 (pages 3111 to 3116)). However, a polyarylene sulfide to be obtained is expected to have no terminal structure or have a small amount of terminal structure if any because it is obtained as an impurity or obtained by a side reaction or the like in polymerization, and a polymer uncertain in terminal structure or terminal amount will be obtained.
Moreover, there is known a method wherein in the conversion of a cyclic polyarylene sulfide into a polyarylene sulfide, various catalyst components to promote the conversion (e.g., a compound with radical generating capability and an ionic compound) are used (e.g., JP 5-301962-A (pages 2 to 6), JP 5-163349-A (pages 3 to 6) and JP 5-105757-A (pages 2 to 4)). Specifically, there has been disclosed a method in which a cyclic arylene sulfide oligomer is thermally ring-opening polymerized in the presence of an ionic ring-opening polymerization catalyst. That method is expected to afford a polyarylene sulfide having a functional group and having a narrow molecular weight distribution. That method, however, is problematic in that much alkali metal will remain in a polyarylene sulfide to be obtained because an alkali metal salt of sulfur such as a sodium salt of thiophenol is used as a ring-opening polymerization catalyst in the synthesis of the polyarylene sulfide. Moreover, there is a problem that a polyarylene sulfide will have an insufficient molecular weight when attempting to reduce the amount of alkali metal remaining in a polyarylene sulfide by reducing the used amount of a ring-opening polymerization catalyst in that method.
As a method of reducing the amount of alkali metal remaining in a polyarylene sulfide, there has been disclosed a method of producing a polyarylene sulfide wherein a cyclic aromatic thioether oligomer is ring-opening polymerized in the presence of a polymerization initiator that generates a sulfur radical on heating (e.g., Specification of U.S. Pat. No. 5,869,599 (pages 27 to 28)). The content of alkali metal in a polyarylene sulfide to be obtained is expected to be reduced because this method uses a nonionic compound as a polymerization initiator. However, the glass transition temperature of a polyphenylene sulfide to be obtained by that method is as low as 85° C. This is because the polyphenylene sulfide to be obtained is low in molecular weight and the polyphenylene sulfide is broad in molecular weight distribution since it contains much lower molecular weight components. Moreover, since the polymerization initiator to be used in that method is lower in molecular weight and inferior in thermal stability as compared with a polyphenylene sulfide, there is a fear that a large amount of gas is generated when the polyphenylene sulfide produced by this method is heated, and molding processability may be poor.
In the ring-opening polymerization in the methods for producing a polyarylene sulfide of JP 5-301962-A (pages 2 to 6), JP 5-163349-A (pages 3 to 6), JP 5-105757-A (pages 2 to 4) and Specification of U.S. Pat. No. 5,869,599 (pages 27 to 28), it is believed that the use of a high purity cyclic polyarylene sulfide oligomer containing substantially no linear polyarylene sulfide as a monomer source is preferable and only a slight amount of linear polyarylene sulfide is allowed to be present. Since a cyclic oligomer is generally obtained in the form of a mixture with a large amount of linear oligomer, a high degree of purification operation is needed to obtain a high purity cyclic body. This increases the cost of a polyarylene sulfide to be obtained.
It could therefore be helpful to provide an industrially useful polyarylene sulfide having narrow molecular weight distribution, having low gassing property, high molecular weight, and high purity, and having functional groups.